1. Field of the Invention
The present invention relates generally to siloxane based resins, and more specifically to a caged conformation organohydridosiloxane composition, methods for the synthesis thereof, and low dielectric constant films formed therefrom.
2. Description of the Related Art
Semiconductor devices often have one or more arrays of patterned interconnect levels that serve to electrically couple the individual circuit elements thus forming an integrated circuit (IC). These interconnect levels are typically separated by an insulating or dielectric film. Previously, a silicon oxide film formed using chemical vapor deposition (CVD) or plasma enhanced CVD (PECVD) techniques was the most commonly used material for such dielectric films. However, as the size of circuit elements and the spaces between such elements decreases, the relatively high dielectric constant of such silicon oxide films is problematic.
In order to provide a lower dielectric constant than that of silicon oxide, dielectric films formed from siloxane based resins are becoming widely used. One such family of films formed from siloxane based resins are the films derived from hydrogen siloxane (HSQ) resins (See, U.S. Pat. No. 3,615,272, Oct. 19, 1971, Collins et al.; and U.S. Pat. No. 4,756,977, Jul. 12, 1988, Haluska et al.) However, while such films do provide lower dielectric constants than CVD or PECVD silicon oxide films and also provide other benefits such as gap filling and surface planarization, it has been found that typically the dielectric constants of such films are limited to approximately 3.0 or greater (See, U.S. Pat. No. 5,523,163, Jun. 4, 1996, Ballance et al.).
As known, the dielectric constant of such insulating films is an important factor where IC""s with low power consumption, cross-talk, and signal delay are required. As IC dimensions continue to shrink, this factor increases in importance. As a result, siloxane based resin materials, and methods for making such materials, that can provide insulating films with dielectric constants below 3.0 are very desirable. In addition, it would be desirable to have a siloxane based resin, and method for making the resin, that provides such low dielectric constant films which additionally have a high resistance to cracking. It would also be desirable for such films to have low stress when formed in thicknesses of approximately 1.0 micron (xcexcm) or greater. It would also be desirable for such a siloxane based resin, and method for making, to provide low dielectric constant films via standard processing techniques. In this manner curing processes that require an ammonia or ammonia derivative type of atmosphere, or other non-standard type of semiconductor process, are avoided.
In accordance with the present invention, organohydridosiloxane resins, and methods for making such resin, are provided. Solutions of such organohydridosiloxane resins are employed for forming caged siloxane polymer films useful in the fabrication of a variety of micro-electronic devices, particularly semiconductor integrated circuits.
The organohydridosiloxane resins of the present invention have the general formulas:
(HSiO1.5)n(RSiO1.5)mxe2x80x83xe2x80x83Formula (1)
H0.4-1.0SiO1.5-1.8)n(R0.4-1.0SiO1.5-1.8)mxe2x80x83xe2x80x83Formula (2)
xe2x80x83H0-1.0SiO1.5-2.0)n(RSiO1.5)mxe2x80x83xe2x80x83Formula (3)
HSiO1.5)x(RSiO1.5)y(SiO2)2xe2x80x83xe2x80x83Formula (4)
wherein:
the sum of n and m, or the sum or x, y and z is from about 8 to about 5000, and m or y is selected such that carbon containing constituents are present in an amount of less than about 40 percent that is between about 0.1 and 40 percent; R is selected from substituted and unsubstituted, normal and branched alkyls, cycloalkyls, aryls, and mixtures thereof; and wherein the specific mole percent of carbon containing substituents is a function of the ratio of the amounts of starting materials. In some embodiments, particularly favorable results are obtained with the mole percent of carbon containing substituents being in the range of between about 15 mole percent to about 25 mole percent.
Polymers in accordance with the present invention have a polymer backbone encompassing alternate silicon and oxygen atoms. In contrast with previously known organosiloxane resins, polymers of the present invention have essentially no hydroxyl or alkoxy groups bonded to backbone silicon atoms. Rather, each silicon atom, in addition to the aforementioned backbone oxygen atoms, is bonded only to hydrogen atoms and/or R groups as defined in Formulae 1, 2, 3 and 4. By attaching only hydrogen and/or R groups directly to backbone silicon atoms in the polymer, unwanted chain lengthening and cross-linking is avoided. Consequently, the shelf life of organohydridosiloxane resin solutions in accordance with the present invention is enhanced as compared to previously known organosiloxane resins. Furthermore, since silicon-carbon bonds are less reactive than silicon hydrogen bonds, the shelf life of the organohydridosiloxane resin solutions in accordance with the present invention is enhanced as compared to previously known hydridosiloxane resins.
In some embodiments, the polymer backbone conformation is a cage configuration. Accordingly, there are only very low levels or reactive terminal moieties in the polymer resin of this invention. This also ensures that no unwanted chain lengthening polymerization will occur in solution, resulting in an extended shelf life. Each silicon atom of the polymer is bonded to at least three oxygen atoms. Moieties bonded to the polymer backbone include hydrogen and organic moieties.
The organic moiety of the organotrichlorosilane monomer is alkyl or aryl and includes, but is not limited to, methyl, ethyl; linear and branched propyl, butyl, pettyl hexyl; and cyclic compounds such as cyclohexyl and phenyl. In some embodiments of the present invention, more than two of the aforementioned staring materials are employed.
In accordance with the method of this invention, synthesis of the organohydridosiloxane composition of this invention includes a dual phase solvent system using a catalyst. In some embodiments of the present invention, the starting materials encompass trichlorosilane and an organotrichlorosilane, for example either an alkyl or an aryl substituted trichlorosilane. The relative ratios of the trichlorosilane and the organotrichlorosilane determine the mole percent carbon-containing substituents in the polymer.
In some embodiments, the method of this invention includes:
1) mixing a solution of hydridotrihalosilanes and organic-substituted trihalosilanes (e.g. trichlorosilane and alkyl or aryltrichlorosilane) to provide a mixture,
2) combining the mixture with a dual phase solvent including a non-polar solvent, and a polar solvent to provide a dual phase reaction mixture,
3) adding a solid phase catalyst to the silane/solvent reaction mixture,
4) reacting the silanes to produce organohydridosiloxanes, and
5) recovering the organohydridosiloxane from the organic portion of the dual phase solvent system.
Additional steps may include washing the recovered organohydridosiloxane to remove any unreacted monomer, and fractionating the organohydridosiloxane product to thereby classify the product according to molecular weight.
In other embodiments, the catalyst is a phase transfer catalyst including, but not limited to, tetrabutylammonium chloride, and benzyltrimethylammonium chloride. The phase transfer catalyst is introduced into the reaction mixture and the reaction is allowed to proceed to the desired degree of polymerization.
In accordance with one aspect of the method of this invention, a dual phase solvent system includes a continuous phase non-polar solvent and a polar solvent. The non-polar solvent includes, but is not limited to, any suitable alkyl or aryl compounds or a mixture of any or all such suitable compounds, the operational definition of xe2x80x9csuitablexe2x80x9d in the present context includes the functional characteristics of:
1) solubilizing the monomeric silicon compounds,
2) solubilizing the resin product,
3) stability of the resin product in the solvent, and
4) insolubility of unwanted reaction products.
Exemplary solvents include, but are not limited to, pentane, hexane, heptane, cyclohexane, benzene, toluene, xylene, halogenated solvents such as carbon tetrachloride, and mixtures thereof.
The second solvent phase is a polar phase, immiscible with the organic, non-polar solvent phase, and includes water, alcohols, and alcohol and water mixtures. It is thought that alcohol solubilizes reactive intermediates that are not yet soluble in the non-polar phase and would ordinarily be unstable in a substantially aqueous phase. The amount of alcohol present is, however, not so high as to significantly dissolve product polymers having molecular weights greater than about 400 AMUs.
Alcohols and other polar solvents suitable for use in the polar phase include, but are not limited to, water, methanol, ethanol, isopropanol, glycerol, diethyl ether, tetrahydrofuran, diglyme and mixtures thereof. In one embodiment, the polar solvent includes a water/alcohol mixture wherein the water is present in an amount sufficient to preferentially solubilize ionic impurities not soluble in alcohol, and/or preclude solvent extraction of product compounds that might otherwise be soluble in alcohol. The polar solvent phase advantageously retains the hydrochloric acid (HCl) condensation product and any metal salt or other ionic contaminants, that may be present. Since any ionic contaminants are retained in the polar solvent phase, the organohydridosiloxane product of this invention is of high purity and contains essentially no metal contaminants.
In accordance with another aspect of the method of the present invention, in one embodiment, a solid phase catalyst, such as Amberjet 4200, or Amberlite I-6766 ion exchange resin (both available from Rohm and Hass Company, Philadelphia, Pa.), surface catalyzes the polymerization of the trihalosilane and organo-trihalosilane monomers into the composition of this invention. Amberjet 4200 is a basic anion exchange resin based on the chloride ion. Amberlite I-6766 is a also a basic anion exchange resin. By way of explanation, and not by way of limitation, it is thought polymer chain propagation occurs on the catalyst surface by hydrolysis of the Sixe2x80x94Cl bond of the monomer to Sixe2x80x94OH, followed by condensation with another Sixe2x80x94OH to provide an Sixe2x80x94Oxe2x80x94Si bond, thereby extending the polymer chain. In other embodiments, polymerization is catalyzed with a phase transfer catalyst such as tetrabutylammonium chloride.
In embodiments of the present invention, the amount of organotrichlorosilane monomer present is an amount sufficient to provide an as-cured dielectric film having an organic content of less than about 40 mole percent carbon containing substituents. Such dielectric films formed in accordance with the present invention advantageously provide low dielectric constants, typically less than 3.0. In some embodiments, particularly alkylhydridosiloxane embodiments, the carbon-containing substituent content is a function of the mole percent alkyl- or aryltrihalosilane.
Additionally, dielectric films in accordance with the organohydridosiloxane composition of this invention exhibit thermal stability permitting cure temperatures up to about 450xc2x0 centigrade.
This invention describes a new class of organohydridosiloxane resins having less than about 40 mole percent carbon-containing substituents. These new resins have improved stability giving longer shelf-life in solution. They also have much lower dielectric constants that are more stable after high temperature treatment than resins with no organic content.
While the invention is described hereinbelow with reference to certain embodiments, it is understood that these embodiments are presented by way of example and not by way of limitation. The intent of the following detailed description is to cover all modifications, alternatives and equivalents as may fall within the spirit and scope of the invention. For example, it is understood that although the examples use a chlorinated silane monomer, other monomers such as trifluorosilane, tribromosilane, organo-trifluorosilane, and organo-tribromosilane may be used. It is also understood that although an ethanol/water solution is typically used as the polar solvent, other alcohols and alcohol/water solutions may also be used.
The following characteristics encompass non-limiting measurements that illustrate the properties of the organohydridosiloxane polymer resins and thin films of the present invention. The methods used in measuring the various characteristics of the organohydridosiloxane resin and polymer films are as follows:
1) Film Thickness (A): Film thickness is measured using a calibrated Nanospec(copyright) AFT-Y CTS-102 model 010-180 Film Thickness Measurement System available from Nanometrics, Co. An average of measurements at five locations on a wafer are reported as the film thickness for each sample.
2) Molecular Weight (xe2x80x9cMWxe2x80x9d): Molecular weight is determined using a gel phase chromatography system from Waters Corporation, Milford, MA, equipped with a Waters 510 pump, Waters 410 differential refractometer and a Waters 717 autosampler. As is the customary practice in the field of silicon polymers, weight average molecular weight is reported. The procedure used is as set forth by S. Rosen in xe2x80x9cFundamental Principles of Polymeric Materials, pages 53-81, (2nd Ed. 1993) and incorporated herein by reference.
3) Dielectric Constant: Dielectric constant is determined using the capacitance-voltage (xe2x80x9cCVxe2x80x9d) measurement technique and employs a Hewlett-Packard Model 4061A semiconductor measurement system at a frequency of 1 MHz. This test procedure employs a metal-insulator-metal (MIM) structure with the thickness of each layer ranging from about 0.3 to 1 micron (xcexcm).
The method of making the compositions of the present invention include, generally, adding a mixture of the organotrihalosilane and hydridotrihalosilane (e.g. trichlorosilane and methyl trichlorosilane) to a mixture of catalyst, non-polar solvent, and polar solvent to form a reaction mixture. The polymerization reaction is allowed to proceed. Upon completion of the polymerization reaction, the reaction mixture is filtered, the polar solvent is separated, and the solution is dried and then evaporated to leave a white solid. This solid may then be slurried in hydrocarbon solvent to remove monomer, and finally evaporated to leave the desired product.
The Mw of the product produced can be varied between 400 and 200,000 AMU depending on the reaction conditions. We have found that materials with molecular weights of 10,000 AMU, 20,000 AMU, 40,000 AMU and 60,000 AMU all have good coating properties.
In one embodiment, the organohydridosiloxane is formulated in a suitable solvent for use as a spin-on-dielectric polymer.